Effect of quercetin on colon contractility and L-type Ca(2+) channels in colon smooth muscle of guinea-pig.

The aim of the present study was to investigate the effects of quercetin on colon contractility and voltage-dependent Ca(2+) channels in the single smooth muscle cell isolated from the proximal colon of guinea-pig and to clarify whether its effect on L-type Ca(2+) current (I(Ca,L)) would be related to its myorelaxing properties. Colon smooth muscle strips were used to take contractile tension recordings. Smooth muscle cells were freshly isolated from the proximal colon of guinea-pig by means of papain treatment. I(Ba,L) (barium instead of calcium as current carrier) was measured by using whole-cell patch-clamp techniques. The results showed that quercetin relaxed colon muscle strips in a concentration-dependent manner and antagonized the contractile effect of acetylcholine and neostigmine. Preincubation with indomethcin [cyclooxygenase (COX) inhibitor] and methylene blue [guanylate cyclase (GC) inhibitor] significantly attenuated the relaxing effect of quercetin, respectively. Quercetin increased I(Ba,L) in a concentration- [EC(50)= (7.59+/-0.38) mumol/L] and voltage-dependent pattern, and shifted the maximum of the current-voltage curve by 10 mV in the depolarizing direction without modifying the threshold potential for Ca(2+) influx. Quercetin shifted the steady-state inactivation curve toward more positive potentials by approximately 3.75 mV without affecting the slope of activation and inactivation curve. H-89 (PKA inhibitor) abolished quercetin-induced I(Ba,L) increase, while cAMP enhanced the quercetin-induced I(Ba,L) increase. The patch-clamp results proved that quercetin increased I(Ba,L) via PKA pathway. It is therefore suggested that the relaxing effect of quercetin attributes to the interaction of GC and COX stimulation, as well as the antagonism effect on acetylcholine, which hierarchically prevails over the increase in the Ca(2+) influx to be expected from I(Ca,L) stimulation.